Regulatory agencies have mandated a reduction in the sulfur, aromatic, and hetero-atom content of commercial diesel and distillate fuels in an effort to improve emissions characteristics of the fuels. This regulatory requirement causes a problem insofar as the fuel industry recognizes that the refining processes needed to produce these fuels require a more severe hydrotreatment which removes polar species from the fuel and reduces its lubricity. Reducing the level of one or more of the sulfur, polynuclear aromatic or polar components of diesel fuel oil can reduce the ability of the oil to lubricate the injection system of the engine, causing the fuel injection pump of the engine to fail prematurely. Even marginally lower lubricity can significantly increase wear of fuel pumps, valves and injector nozzles over an extended period of use.
The problem of poor lubricity in these fuels is likely to be exacerbated by future engine system developments aimed at further decreasing emissions. This will result in an increase in the fuel oil lubricity requirement relative to requirements for present engines. For example, the use of high pressure unit injectors will likely increase the need for better fuel oil lubricity.
For the reasons above, there has been an ever-growing effort to produce additives, which can improve the lubricity of fuels low in sulfur and/or aromatics. For example, dimertrimer acids are sold commercially as lubricity additives. Moreover, commercially available tall oil fatty acids are used as lubricity improvers for low sulfur and/or aromatic fuels. A problem associated with additives based on acids is their tendency to cause gel formation in the fuel filter, due to an incompatibility with other lube oil additives into which they may come into contact. For this reason, some fuel producers specify non-acidic chemistries to avoid these problems.
To avoid the problems associated with acid groups, a number of lubricity improvers are available commercially which have been produced by reacting the acid to form an amide or ester. For example, U.S. Pat. Nos. 4,789,493 and 4,808,196 to Horodysky describe N-alkylalkylenediamine amides and their use as friction reducing additives in lubricants. As described, these additives are preferably made by reacting an N-alkylalkylenediamine with a carboxylic acid. One problem associated with such amide additives is that the reactions used for their formation can be reversed, causing a regeneration of the acid which leads to the same gellation problem in fuel filters encountered as when acidic lubricity improvers are added. In addition, these additives have a tendency to cyclize over time.
U.S. Pat. No. 5,492,641 and EP 0568873 B1 to Mohr disclose the use of Beta-aminonitriles, and/or N-alkylpropylenediamines obtained by hydrogenating these Beta-aminonitriles, as detergents and dispersants in gasoline fuels. It further discloses that these compounds may be used as lubricant additives for gasoline fuels.
U.S. Pat. No. 3,677,726 describes the use of monosubstituted ureas as varnish-removing fuel additives for hydrocarbon fuel compositions. It is known that fuel oils are prone to form gum during periods of prolonged storage that result in formation of resin-like deposits on equipment, such as fuel lines and filters that can be problematic. The additives are disclosed as being effective in removing lacquer and varnish deposits attributable to gum after they have formed.
U.S. Pat. No. 3,615,294 to Von Allmen and U.S. Pat. No. 3,762,889 to Newman et al. each describe a gasoline fuel composition containing a carburetor detergent additive comprised of the neutral salt reaction product of a substituted urea and a paraffinic oil oxidate. The substituted urea can be formed from the reaction between commercial Duomeen (N-alkyldiaminopropane) and urea. The paraffin oil oxidate, as known in the art, corresponds to a large and poorly defined group of various types of chemical functionalities that are formed when oxidizing a base lubricating oil. This reference does not disclose the use of either the salt adduct of the substituted urea or the substituted urea itself as lubricity enhancers, nor does it disclose the use of either of these compounds in fuels outside those consisting of a mixture of hydrocarbons in the gasoline boiling range (i.e. from about 75° C. to 450° C.).
As described above, the poly-aromatic content of a fuel has dramatic effects on the lubricity of a fuel. Since gasoline and diesel have very different amounts of aromatic content, it would be expected that the effects of an additive would behave differently for each of these fuels. In addition, lubricity is generally not a problem in a gasoline-based engine because the fuel pump is lubricated by crankcase oil. In this situation, the lubriciousness of the fuel is not an issue when considering fuel pump wear. For a diesel engine, however, the situation is quite different. In these engines, the lubrication of the fuel pump is accomplished by the diesel fuel itself.
There is a growing need in the fuel industry for stable, non-acidic compounds which can serve as lubricity improvers for fuels treated to be low in sulfur and/or aromatic components, particularly given the increasing pressure from regulatory agencies to produce such fuels worldwide. In particular, there is a need for lubricity additives for low sulfur diesel fuels.